Is that because the corner sub is far too low compared to the rest? For example, I've never seen centering 9.5 corner 7.5 edge 9.5 surface 9.5 gets a 8.5 overall.

Corners aren't weighed differently than any other sub.
And it isn't true that to get a 9.5 you can't have a sub below 9. You can only get a 9.5 with an 8.5 if the other 3 subs are 10. Very rare to come by.